Chemical storage advance may enable more cost-effective concentrated solar-power storage

New thermochemical energy storage system is twice as efficient, with 10 times higher energy density
November 4, 2015

An advance in the storage of concentrated solar thermal energy may reduce reduce its cost and make it more practical to supply 24-hour on-demand electrical power (credit: Kelvin Randhir, courtesy of the University of Florida)

Oregon State University (OSU) engineers have developed an innovation in chemical storage of concentrated solar thermal energy that may reduce its cost and make it more practical for wider use.

The new system uses thermochemical storage, in which chemical transformation is used in repeated cycles to hold heat, use it to drive turbines to create electricity, and then be re-heated to continue the cycle. Most commonly, this might be done over a 24-hour period, with variable levels of solar-powered electricity available at any time of day, as dictated by demand.

Unlike conventional solar photovoltaic cells, concentrated solar thermal (a.k.a. concentrated solar power, or CSP) uses huge arrays of mirrors to focus light, typically onto a tower, for temporarily storing the energy, which is more cost-effective than batteries. (See Australian researchers set new world record in solar-energy efficiency.)

The PS10 Solar Power Plant in Spain concentrates sunlight from a field of  624 heliostats (movable mirrors) onto a central solar power tower, generating 11 megawatts (credit: Abengoa Solar, S.A.)

Storage of this type helps eliminate one of the key factors limiting the wider use of solar energy: The need to deliver the electricity immediately.

A ten-fold increase in energy density and twice as efficient

The OSU development overcomes a limitation in thermochemical energy storage. “In these types of systems, energy efficiency is closely related to use of the highest temperatures possible,” said Nick AuYeung, an assistant professor of chemical engineering in the OSU College of Engineering and corresponding author of a paper in ChemSusChem, a professional journal covering sustainable chemistry.

Thermochemical storage functions like a battery, in which chemical bonds are used to store and release heat (not electrical) energy.  However, the molten salts now being used to store solar thermal energy can only work at about 600 degrees centigrade, and also require large containers and corrosive materials, he explained. “The compound we’re studying can be used at up to 1,200 degrees, and might be twice as efficient as existing systems. There’s a significant potential to lower costs and increase efficiency.”

AuYeung said the new OSU system is based on the reversible decomposition of strontium carbonate into strontium oxide and carbon dioxide, which consumes thermal energy. During discharge, the recombination of strontium oxide and carbon dioxide releases the stored heat. These materials are nonflammable, readily available, and environmentally safe.

In comparison to existing thermochemical approaches, the new system could also allow a ten-fold increase in energy density (energy storage per unit volume), and it’s physically much smaller and would be cheaper to build. The proposed system could first be used to directly heat air, which would drive a turbine to produce electricity, and then residual heat could be used to make steam to drive yet another turbine.

However, in laboratory tests, the current energy storage capacity of the process declined after 45 heating and cooling cycles, due to some changes in the underlying materials. Further research will be needed to identify ways to reprocess the materials or significantly extend the number of cycles that could be performed before any reprocessing was needed, AuYeung said.

Other refinements may also be necessary to test the system at larger scales and resolve issues such as thermal shocks, he said, before a prototype could be ready for testing at a national laboratory.

The work was supported by the SunShot Initiative of the U.S. Department of Energy, and done in collaboration with researchers at the University of Florida.

Abstract of Solar Thermochemical Energy Storage Through Carbonation Cycles of SrCO3/SrO Supported on SrZrO3

Solar thermochemical energy storage has enormous potential for enabling cost-effective concentrated solar power (CSP). A thermochemical storage system based on a SrO/SrCO3 carbonation cycle offers the ability to store and release high temperature (≈1200 °C) heat. The energy density of SrCO3/SrO systems supported by zirconia-based sintering inhibitors was investigated for 15 cycles of exothermic carbonation at 1150 °C followed by decomposition at 1235 °C. A sample with 40 wt % of SrO supported by yttria-stabilized zirconia (YSZ) shows good energy storage stability at 1450 MJ m−3 over fifteen cycles at the same cycling temperatures. After further testing over 45 cycles, a decrease in energy storage capacity to 1260 MJ m−3 is observed during the final cycle. The decrease is due to slowing carbonation kinetics, and the original value of energy density may be obtained by lengthening the carbonation steps.